I would think a T42p should work with 1TB mSATA -- I've got an older A31p running with 500gb mSATA (1TB was too much) and a newer T43pS with a 2TB SSD -- so, 500gb should be safe bet.

Also, for more storage, consider you can put another HDD/SSD in the UltraBay (probably up to 2TB), and/or spare unused USB port(s) could be fitted with affordable SanDisk UltraFits which run up to 256gb and barely stick out at all (e.g., I use one for a full music library).

Just curious, so did your Samsung Evo 860 SSD of 1 TB capacity work on your T42p ?
I suspect your 1 TB SSD is a SATA interface SSD, so would not fit into the Parallel ATA (PATA) interface of T42p.

The T42/T42p era used Parallel ATA (PATA) Hard Drive, and back then, even a 120 GB Hard Drive is very large back then.
I did use a 120 GB 7200 RPM HDD at one point for my T42 (which is not T42p), then upgraded to a similar size Transcend PATA SSD.

(The numerous very-short system freeze-up under Win XP when using this Transcend PATA SSD scared the ** out of me.
But from actual 1+ year of using this PATA SSD, there was no data corruption from those very-short system freeze-up.)

[Edit: Sorry, just saw your above Jan'2019 forum posting for the first time today in March'2019]